Digital subscriber line (DSL) systems are becoming increasingly popular for providing a user broadband access to locally or globally interconnected networks such as the internet. A DSL connection is using subscriber lines which are typically provided by twisted pair copper to establish a bidirectional electrical transmission between a transceiver unit located at a provider's site of a subscriber line and a transceiver unit located at the subscriber's site
In order to establish a data transfer, both transceiver units at the end of the subscriber lines have to perform a predefined initialization sequence. During the initialization, channel parameters are analyzed and information such as profiles, settings and control parameters etc is exchanged or determined. Training sequences for training operation parameters such as FEXT (far-end cross talk) coupling coefficients, NEXT coefficients (near-end cross talk) coupling and echo cancellation coefficients may be also executed during initialization.
After initialization, the transceiver units are entering showtime operation mode where user data is transferred between the two transceiver units. Both of the transceiver units are maintained activated in order to perform transmission and receiving of data.
After finishing a session, the transceiver unit may be shut down until another session starts. For the next session, both transceiver units have to undergo again the time-consuming initialization procedure as described above.
For some applications, such as for example VoIP (voice over IP), data connection between the transceiver units has to be established in short times since users are expecting to receive incoming telephone calls with no or only short time delays.
If both transceiver units are maintained all time activated for data transmission (“always-on”) in order to allow instant data connections, they have to be operated with full-power which increases the overall energy consumption of the system significantly. Furthermore, the “always-on” operation may cause heat problems for example in small cabinets and may decrease the lifetime of system components.
In some DSL standards such as ADSL 2 according to ITU-T/G.992.3 and ADSL 2 plus according to ITU-T/G. 992.5 power saving states have been introduced in order to address the above problem. Different power management states L0, L2 and L3 are provided, wherein L0 is a full-power mode as used in showtime data transmission, L2 is a power saving mode wherein data transmission is provided with lower power spectral density and therefore reduced data transmission rate and L3 is a power saving state wherein the transmitter applies zero voltage to the subscriber line and the transceiver units are basically allowed to enter a sleep mode. L2 power management state may be entered and exited based on the current data transmission rate. For example, when the user loads large data files from the internet, the transceiver unit may be maintained in the L0 power management state while when the data transmission rate is lowered, for example when the user is busy with other activities such as writing messages or reading documents on the computer, the transceiver unit enters the L2 power management state. While in L2, the DSL system can instantaneously enter the L0 state allowing for full-rate data transmission. The L2/L0 entry/exit is achieved without any service interruption and is basically not noticeably for a user.
The L3 power management state can be considered as a sleep mode wherein no user data is transmitted between the two transceiver units. Nevertheless, when the user decides to again establish a data connection or when a VoIP call is incoming at a transceiver unit, an initialization or re-initialization is necessary for providing the data connection. Typically, the initialization procedure requires about 10 to 30 seconds.
In order to reduce the initialization time period, the ADSL 2 plus standard describes a short initialization procedure allowing an initialization of about 3 seconds from L3 to L0 which may be depending on the implementation also longer. However, for applications such as triple play applications like VoIP or video over IP, data transmission has to be established in time periods as short as 1 second or less. Existing DSL systems are not able to establish such short transitions.